To finish an annular workpiece it is standard to hold it in an inside or outside chuck, depending on whether the outside or inside surface is to be machined, and to rotate it while pressing a grinding stone against the surface to be machined. To carry off particles and cool the grinding process it is standard to supply a cutting liquid to the site.
The workpiece is typically held in a chuck to rotate it relative to the nonrotating tool. If the inside periphery is being ground, the chuck engages the outside periphery, typically with three jaws to keep the workpiece on center. Conversely if the outside periphery is being ground the chuck engages the inside periphery, also with three jaws that bear radially outward.
It is necessary to monitor the grinding operation by taking periodic measurements of the surface being ground. Typically the radius of curvature of the surface being ground is continuously or intermittently measured and this measurement is compared in a control computer as an actual value against a set point representing the desired radius of curvature, and grinding is continued or stopped according to whether or not the desired size is attained.
These measurements are typically taken by means of feelers which mechanically touch the surface being machined with a tiny diamond point. Not only can such a procedure mar the surface, but it is also possible for the feeler to displace the workpiece in the chuck or even deform the workpiece slightly. In addition the workpiece inherently has microscopically eccentric regions that create vibrations in the measuring device that in turn give false readings.
In other machining operations contactless measurements are proposed. For instance a compressed-air jet can be directed from a nozzle at a surface being machined to create a restriction between the nozzle tip and the workpiece. The nozzle tip is held stationary so that the flow cross section of the restriction changes proportionately with the spacing between the nozzle tip and the workpiece surface. The pressure in the nozzle can be measured to determine workpiece spacing. Such a procedure is not normally extremely sensitive so it is unusable in, for instance, machining of bearing races.
It is also known when machining a flat surface to use a nozzle as described above, but emitting a continuous liquid stream, and to monitor liquid pressure as analogous to nozzle/surface spacing. Such a procedure cannot work on nonflat surfaces. Furthermore like the air-nozzle system, such an arrangement exerts a perceptible force against the workpiece which can displace it unless it is solidly clamped.